Recently in office chairs used in working spaces or meeting rooms of an office, the back of a user is supported by the backrest over which elastic covering material or a tension member is stretched. In the backrest of the chair, a front surface of a back frame molded in a closed loop of metal such as Al which can easily be molded or synthetic resin is covered with the tension member. The circumference of the tension member is attached to the back surface or side of the back frame and the tension member is stretched over the back frame in Patent Literature 1.
Meanwhile, there is a great demand for a cantilever-type chair in which the backrest is connected to a leg frame made of pipes. The chair provides the armrest at part in which the backrest is connected to the leg frame in Patent Literatures 2-4.
In the chair with the armrest, the middle of the back frame is connected to the rear ends of the armrests. The seat is supported by a seat support in the middle of front legs of the leg frame.